Generally the most important requirement for continuous welders is to be fed in so that the highest continuity and regularity may be reached.
Also and above all in welders of the above mentioned kind this requirement is of fundamental importance: in fact a small increase or decrease in the welding rate would involve the melting of the material to be welded or a non-welding respectively.
In the case of continuous welders for cylindrical box-shaped elements usually called cans, the tin plates that will form the cylindrical part of the cans are fed in along a horizontal arm. Along said arm the tin plates are first calendered then they are caused to advance by a toothed entraining chain until they are under a fork-shaped element. This fork-shaped element removes the cans from the entraining teeth of the chain and pushes them into a shaping device until they are in engagement with two disk electrodes.
The shaping device comprises several sets of shaped rollers disposed on parallel circumferences and one of said sets, usually the last, consists of gauged rollers the inside of which defines a circumference having the same diameter as the cans to be shaped.
Therefore the two disk electrodes simultaneously carry out the welding and cause the cans to advance towards a pair of entraining belts the task of which is of moving them away. As a result, the setting of the disk electrodes, which have to fulfil the two above mentioned functions, is very difficult and subjected to variations. As already said, these variations may give rise to the melting of the material of which the tin plates are made if there is a slackening of speed and therefore the tin plates stop longer than necessary between the electrodes, or a non-welding of the edges if they move at too high a speed thereby stopping for a shorter time than the required one.
In both cases the cans do not appear completely welded and the product contained therein might be subjected to deteriorations that in the case of food could even be lethal.